Relaxes regex for removing duplicate slashes;

Prior to this, duplicate slashes would be removed unless preceeded by "http" or "https" protocols. This was too strict.
The regex was updated to allow for any 2+ character alphanumeric protocol. Went with requiring at least 2 characters since every scheme listed here (https://en.wikipedia.org/wiki/List_of_URI_schemes) is at least 2 characters.

index.js

@@ -117,7 +117,7 @@ const normalizeUrl = (urlString, options) => {
 
 	// Remove duplicate slashes if not preceded by a protocol
 	if (urlObj.pathname) {
-		urlObj.pathname = urlObj.pathname.replace(/(?<!https?:)\/{2,}/g, '/');
+		urlObj.pathname = urlObj.pathname.replace(/(?<!(?:[a-z0-9]{2,}):)\/{2,}/g, '/');
 	}
 
 	// Decode URI octets

test.js

@@ -205,6 +205,13 @@ test('remove duplicate pathname slashes', t => {
 	t.is(normalizeUrl('http://sindresorhus.com///foo'), 'http://sindresorhus.com/foo');
 	t.is(normalizeUrl('http://sindresorhus.com:5000//foo'), 'http://sindresorhus.com:5000/foo');
 	t.is(normalizeUrl('http://sindresorhus.com//foo'), 'http://sindresorhus.com/foo');
+	t.is(normalizeUrl('http://sindresorhus.com/s3://sindresorhus.com'), 'http://sindresorhus.com/s3://sindresorhus.com');
+	t.is(normalizeUrl('http://sindresorhus.com/s3://sindresorhus.com//foo'), 'http://sindresorhus.com/s3://sindresorhus.com/foo');
+	t.is(normalizeUrl('http://sindresorhus.com//foo/s3://sindresorhus.com'), 'http://sindresorhus.com/foo/s3://sindresorhus.com');
+	t.is(normalizeUrl('http://sindresorhus.com/git://sindresorhus.com'), 'http://sindresorhus.com/git://sindresorhus.com');
+	t.is(normalizeUrl('http://sindresorhus.com/git://sindresorhus.com//foo'), 'http://sindresorhus.com/git://sindresorhus.com/foo');
+	t.is(normalizeUrl('http://sindresorhus.com//foo/git://sindresorhus.com//foo'), 'http://sindresorhus.com/foo/git://sindresorhus.com/foo');
+	t.is(normalizeUrl('http://sindresorhus.com/a://sindresorhus.com//foo'), 'http://sindresorhus.com/a:/sindresorhus.com/foo');
 });
 
 test('data URL', t => {